Inorg. Chem. 1991, 30, 3234-3236
3234
rather than by its polarity. The influence of the solvent basicity is reduced when the substituent R becomes more bulky. Because of the basic properties of water, these results imply that a hydrophobic environment and protection from solvent attack are indeed essential factors for the stabilization of the [4Fe-4SI3+ core in H P proteins. Another conclusion may be that a low-molecular-weight thiolate cluster compound when dissolved in a lowdonor-number solvent like N B or Dc can be regarded to be a good model system for the active site of H P proteins. Acknowledgment. We thank A. M. Roelofsen for the recording of the cyclic voltammograms in benzonitrile. This investigation was supported by the Netherlands Foundation for Chemical Research (SON),with financial aid from the Netherlands Organization for Scientific Research (NWO), and by the Centre National de la Recherche Scientifique-DRCI. R@try NO. AN, 75-05-8;BN, 100-47-0;ACN, 67-64-1;DC, 75THF, 109-99-9;DMSO, 67-68-5; 09-2;DMF, 68-12-2;NB, 98-95-3; [Fe4S4(SPh)4]2-,52325-39-0;[ Fe4S4(S-f-B~)4]2-, 5 1913-87-2;[ Fe4S4(SEt)J2-, 52261-51-5; [ Fe4S4(S-n-Pr)4]2-, 52325-40-3;[ Fe4S4(SCH2Ph),lZ-, 52349-82-3;[Fe4S4(S-p-C6HqC1)4]2-, 55787-38-7; [ Fe4S4(S-p-C6H4CH3)4]z-,5 1899-68-4;[ Fc4S4(SCH2CH2OH),I2-, 62851-99-4;[Fe4S4(SPh),l-,97365-72-5; [Fe4S4(S-r-Bu)4]-, 126752-32-7; [ Fe,S,(SEt),]-, 134566-32-8;[Fe,S,(S-n-Pr),]-, 134566-33-9; [Fe4S4(SCHZPh),]-, 134593-43-4;[F~,S,(S-~-C~H,CI)I]-,134566-34-0; [ Fe4S4(S-p-C6H,CH’),]-, 134566-35-1 ; [ Fe4S4(SCH2CHz0H)4]-, 134566-36-2; Pt, 7440-06-4;[Fe,S,(SPh),]’-, 52627-89-1;[Fe,S,(S-r91294-54-1;[Fe4S4(SCH2CH20H)4]p,134566-37-3; [Fe,S,(SBU),]~, p-C6H4CHJ4]’-, 67724-72-5;[Fe4S4(SCH2C6H~),]’-,63 138-11-4; [ Fe,S4(SEt)4]3-,52499-30-6;[ Fe4S4(SPh),],134566-38-4;[Fe,S,(S-fBu),], 134566-39-5. (25) Kadish, K. M.; Das, K.; Schacper, D.; Merrill, C. L.; Welch, B. R.; Wilson, L. J. Inorg. Chem. 1980, 19, 2816.
Table I. Crystal Data for the Tetraphenylarsonium Salt of the 1.4-Dicvanamido-2.3.5.6-tetramethvlbenzeneDianion IAsPhAldL1) formula As2N4CSOHS2MW 966.84 space group P 2 , / n Z 2 cryst syst monoclinic De; g/cm3 1.323 a, A 12.6115 (24) temp, O C 22 b, 8, 14.3029 (17) radiation (A, A) Mo (0.70930) c, A 13.6986 (14) R facto? 0.051 fl, deg 100.820(10) Rw factorb 0.051 v,A’ 2427.0 (6)
-0.9 ~ m - l . The ~ authors pointed out that a t this magnitude of coupling it would be difficult to distinguish between intermolecular and intramolecular mechanisms. In contrast, a later study found moderately strong intramolecular antiferromagnetic coupling (W = -140 cm-I) between Cu(I1) ions separated by 11.25 A in [X2Cu2(0H2)(p-terephthalato)][C1O4I2,where X = 1,4,7-trimethyl- 1,4,7-triaza~yclononane.~ The difference in magnetic coupling between these two results is clearly dependent on the nature of the interaction of the magnetic orbitals with the bridging ligand. For maximum resonance exchange of the magnetic orbitals,’~’the bridging ligand should possess a HOMO delocalized on its donor atoms with the correct symmetry and energy to optimize its interaction with the magnetic orbitals. Even if the bridging ligand’s HOMO has improper symmetry to interact with the magnetic orbitals, the spin polarization mechanism can still induce antiferromagnetic coupling.’** The possibility that long-range antiferromagnetic coupling might occur between magnetic orbitals bridged by an easily oxidized, extended r HOMO system led us to prepare the dinuclear complex [~L-L((NH,),RU)~] [C1O4I4 (l), where Lz- = 1,Cdicyanamido2,3,5,6-tetramethylbenzenedianion. The physical characterization of this complex, including its temperature-dependent magnetic properties, is the subject of this report.
Experimental Section Contribution from the Ottawa-Carleton Chemistry Institute, Carleton University, Ottawa, Canada K l S 5B6, Chemistry Division, National Research Council of Canada, Ottawa, Canada KIA OR6, and Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Long-Range Antiferromagnetic Coupling between Two Ruthenium(II1) Ions Bridged by a 1,4JXcyanamidobenzene Dianion Ligand Manuel A. S.Aquino,’ Florence L. Lee,: Eric J. Gabe,*
J. E. Greedan,i and Robert J. Crutchley**+ Received July 24, 1990 The study of polymetallic complexes in which coupling between metals is propagated via a bridging molecule has clear application to the design of novel magnetic and electronic solid-state materials and to the role of polymetallic sites in biological processes.’** A fundamental property to be understood is the distance dependence of metal-metal interactions. Researched have proposed the relation 12Jliml= 1.35 X lo7 exp(-1.80R)
(1)
for the limiting value of J (cm-’)? the magnetic exchange coupling constant, and R (A), the distance between metal ions. In approximate agreement with eq 1, a study of the distance dependence of the magnetic coupling between two Cu(I1) com lexes bridged by 4,4’-bipyridine (metal ions separated by 1 1.1 ) found 2J =
x
+ Carleton
University.
*NationalResearch Council of Canada. 1 McMaster University.
0020-1669/91/1330-3234$02.50/0
Physical Measurements. The equipment used to perform cyclic voltammetry and UV-vis spectroscopy has been described in a previous paper.9 Temperature-dependent magnetic measurements were performed on a Quantum Design SQUID magnetometer from 5 to 300 K in a 1.O-T field. Elemental analyses were performed by Canadian Microanalytical Services Ltd. Materials. All solvents and solid chemicals were reagent grade or better. [(NH,)SRu(OH2)][PF6I2was prepared by literature methods.I0 Protonated 1,4-dicyanamido-2,3,5,6-tetramethylbenzene was prepared from its thiourea precur~or.~~” Preparation of Bis(tetrapbenylarsonium) 1,4-Dicyanrmido-2,3,5,6tetramethylbenzene(2-) ([AsPh,ML]). The protonated ligand LHz (0.68 g) and NaOH (3 g) were placed in a 100-mL round-bottom flask and purged with argon. A previously degassed 30-mL aliquot of water was transferred to the reaction flask under argon. The mixture was stirred until complete dissolution. Tetraphenylarsoniumchloride monohydrate (2.6g) was dissolved into 30 mL of 2.5 M NaOH aqueous solution. This solution was degassed and then transferred under argon to the basic solution of the ligand. The resulting yellow precipitate was filtered out, ~~
~
(1) Willet, R. D.,Gatteschi, D., Kahn, O., Eds. Magnero-Srrucrural Correlations in Exchanged Coupled Systems; Reidel: Dordrecht, Holland,
1985. (2) Marcus, R. A.; Sutin, N. Biochem. Biophys. Acra 1985, 811, 265. (3) Coffman, R. E.;Buettner, G. R. J. Phys. Chem. 1979,83. 2387. (4) Here the magnetic interaction between spins S, and Sbfor atoms A and B is of the form H = -US,&. ( 5 ) Julve, M.; Verdaguer, M.; Faus, J.; Tinti, F.;Moratal, J.; Monge, A.; Gutierrez-Puebla, E. Inorg. Chem. 1987, 26, 3520. (6) Chaudhuri, P.; Oder, K.; Wieghardt, K.;Gehring, S.;Haasc, W.;Nuber, B.; Weiss, J. J. Am. Chem. Soc. 1988, 110, 3651. (7) Tinti, F.;Verdaguer, M.; Kahn, 0.; Savariault, J.-M. Inorg. Chem. 1987, 26, 2380. (8) Rosen, R. K.; Anderson, R. A,; Edelstein, N. M. J. Am. Chem. Soc. 1990, 111, 4588. (9) Crutchley, R. J.; Naklicki, M. L. Inorg. Chem. 1989. 28, 1955. (10) Callahan, R. W.;Brown, G. M.; Meycr, T. J. Inorg. Chem. 1975, 14, 1443. (11) Aumuller, A,; Hunig, S. Liebigs Ann. Chem. 1986, 142.
0 1991 American Chemical Society
Inorganic Chemistry, Vol. 30, No. 16, 1991 3235
Notes
A
Table 11. Significant Atomic Parameters" and Ebb Values for
[AsPh,l&l N1 N2 C1 C2 C3 C4 C5 C6
V I
I
- 0.55
0.0
+0'5 V vs.NHE
+
1.25
X
Y
2
Ewe
0.5471 (6) 0.5952 (6) 0.5983 (7) 0.5238 (7) 0.4283 (6) 0.7062 (6) 0.5725 (7) 0.3520 (6)
0.4492 (4) 0.5569 (5) 0.5327 (5) 0.4770 (5) 0.4440 (5) 0.5625 (6) 0.5082 (7) 0.3826 (5)
0.3157 (6) 0.1903 (6) 0.4743 (6) 0.4079 (6) 0.4337 (6) 0.4483 (6) 0.2534 (7) 0.3582 (5)
4.9 (4) 6.1 (4) 4.0 (4) 3.9 (4) 3.7 (4) 5.2 (5) 4.3 (5) 4.6 (4)
"The atom number scheme is given in Figure 2. *Estimated standis the mean of the principal ard deviations are in parentheses. axes of the thermal ellipsoid in A2.
Figure 1. Cyclic voltammogram of 1 in 0.1 M NaCl aqueous solution taken with a glassy-carbon working electrode. Scan rate = 100 mV s-l.
washed with ice cold water, and then vacuum dried. The final product was air sensitive and hygroscopic. Yield: 2.1 g (68%). Prep.ration of [p-L{(NH3)5Ru)21Clo41, (1). Under argon, in 80 mL of acetone, 0.49 g (0.5 mmol) of [AsPh,12[L] and 0.54 g (1.2 mmol) of [(NH3)5Ru(OH2)][PF6]2 were permitted to react for 2 h. After air oxidation and filtration, the crude product was isolated as a bromide salt by the addition of 2.42 g (7.5 mmol) of tetrabutylammonium bromide and then purified by ion-exchange chromatography by using Sephadex C25-I20 resin and eluting with 1 M NaCl solution. The dinuclear complex was precipitated from the eluent by the addition of NaC104and then recrystallized by ether diffusion into an acetone solution, affording dark green-blue crystals of complex 1, [p-L((NH3)5Ru]2][C!04142/3CH3COCH3.Yield: 78 mg (1 1%). The presence of acetone in the product was verified by IR spectroscopy. Anal. Calcd for C14H~N,4016,&14R~2: C, 16.47; H, 4.53; N, 19.21. Found: C, 16.75; H, 4.36; N, 19.61. X-ray Diffraction Stdies. A summary of crystal data for [AsPh412[LI is given in Table 1. Deep yellow crystals of the dianion ligand were grown by allowing a concentrated warm dimethyl sulfoxide solution to cool to room temperature. The diffraction intensities were collected on a Nonius diffractometer with Mo Ka radiation by using the 8/20 scan technique with profile analysis.12 Unit cell parameters were obtained by least-squares refinement of the setting angle for 22 reflections (40 < 20 < 45). Lorentz and polarization factors were applied, but no corrections were made for absorption. The structures were solved by direct methods and refined by fullmatrix least squares. Hydrogen atom pitions were calculated. All the calculations were performed with the NRCVAX Crystal Structure Package.') The atomic parameters, anisotropic thermal parameters, final structure factors, and complete bond lengths and bond angles are available as supplementary material.
Results and Discussion The UV-vis spectra of mononuclear pentaammineruthenium-
(HI)complexes of phenylcyanamide anion ligands have been investigated, and assignments have been made to the major feat u r e ~ ? . ' ~In these complexes, intense r - ~ transitions * are found in the UV region and two LMCT bands are found in the visible region. The LMCT bands arise out of the u interaction between Ru(II1) and the two lone electron pairs of u symmetry on the cyanamide anion group. The dinuclear ruthenium(II1) complexes of 1,4-dicyanamidobenzene dianion ligands are expected to have the same basic features as their mononuclear analogues. In this regard, the major bands found in the absorption spectrum of complex 1 are intense phenyl ring PA* bands a t 213 (38000), 252 (14450), and 353 nm (3160 M-' cm-I) and two ligand to metal charge-transfer bands at 465 (3 160) and 795 nm (8 130 M-' cm-I). The cyclic voltammogram of free Lz-in acetonitrile solution15 Grant, D.F.;Gabe, E. J. J. Appl. Crysrallogr. 1978, 11, 114. Gab. E.J.; Lee, F.L.; Lepage, Y. J. Appl. Crysrallogr. 1989.22. 384. (14) Cmtchley, R.J.; McCaw, K.;Lee. F. L.; Gabe, E.J. Inorg. Chem. 1990, (12) (13)
29. 2576. ., - -.
(15) Cyclic voltammetry of [AsPh&[L] was performed in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphateelectrolyte and with a latinum working electrode at 25 OC and at a scan rate of 100 mV
P
s- .
C(4)
Figure 2. ORTEP drawing of the dianion ligand, L2-. Selected bond lengths (A) and bond angles (deg): N2-C-5, 1.187 (14); C5-N1, 1.283 (14); Nl-C2, 1.410 (11); C2C1, 1.416 (12); CI-C3,, 1.405 (12); C1C4, 1.531 (12); C2C3, 1.403 (12); C3-C-6, 1.547 (10); N2C5-N1, 174.6 (9); C5-N1-C-2, 121.6 (7); NlC2-Cl, 120.8 (8); C2-Cl-C3,, 117.8 (8); C2-C3Cla, 120.4 (7).
shows two reversible one-electron waves at -0.565 and -0.070 V vs NHE, corresponding to the L(-/2-) and L(O/-) couples, respectively. The cyclic voltammogram of 1 is shown in Figure 1.I6 In previous studies of mononuclear pentaammineruthenium(II1) phenylcyanamide c~mplexes?.'~the Ru(III/II) couple was determined to range from approximately 0 to -0.3 V vs NHE, depending on the nature of the anionic phenylcyanamide ligand. It therefore seems likely that the large wave centered at -0.083 V in Figure 1 corresponds to the Ru(III/II) couple. The two smaller waves at 0.475 and 0.822 V correspond to the one-electron ligand redox couples, anodically shifted from their free ligand values because of the u interaction between the Ru(II1) ions and the cyanamide bridging ligand.g The peak current for the Ru(III/II) redox wave is approximately twice as large as that for the one-electron ligand redox waves. In addition, the anodic and cathodic wave separation (E, - Ep = 72 mV) of the Ru(III/II) couple is greater than that seen for the mononuclear pentaammineruthenium(II1) complexes (60-65 mV a t a scan rate of 100 mV s-I).l4 Similar peak shapes in weak electronically coupled dipyridyl-bridged bis(pentaammineruthenium) complexes" are the result of two successive one-electron processes being so close in proximity that only a single wave can be resolved. The probable structure of 1 can be derived by using information from the crystal structures of [(NH3)sRu(2,3-dichlorophenylcyanamidol [SO4]I4and free L2-. The crystal structure of the mononuclear Ru(II1) complex showed that the Ru(II1) ion binds to the cyano nitrogen of the cyanamide group with a bond angle of 17lo.l4 Crystal data and atomic positional parameters for significant atoms of [AsPh412[L] are given in Tables I and 11, respectively, and Figure 2 illustrates the structure of L2- with important bond lengths and angles included in the figure caption. The structure of L2- reveals that the approximately linear cyanamide anion groups are in an anti configuration and clearly out of the phenyl ring plane. It is not unreasonable to suggest that 1 would adopt the most stable conformation of the bridging ligand in the solid state.I8 The through-space distance between Ru(II1) (16) Cyclic voltammetry of 1 was performed in aqueous solution with 0.1 M NaCl and with a glaay-carbonworking electrode at 25 OC and at a scan rate .-..of . . 100 ...m ...V s-1. . (17) Sutton, J. E.;Sutton, P.M.; Taube, H. Inorg. Chem. 1979, 18, 1017. Kim, Y.;Lieber, C. M. Ibid. 1989, 28, 3990.
Inorg. Chem. 1991, 30, 3236-3238
3236 0.7I
I
in the following schematized drawing of its
HOMO: Y
01 0
100.
200.
300
Temp(K)
Figure 3. Molar magnetic susceptibility from 5 to 300 K in a 1.0-Tfield for 1. The solid line is a fit to eq 2 plus a CuritWeiss term, which yields the parameters given in the text.
ions is then estimated to be 13 A. Temperature-dependent magnetic susceptibility data for 1 are plotted in Figure 3 and show a broad maximum at about 90 K. This feature is characteristic of intramolecular antiferromagnetic exchange. Intermolecular antiferromagnetic couplings normally result in a phase transition to a long-range ordered state, which gives a sharp maximum a t the transition temperature (Nee1 temperature). Intermolecular exchange has been studied in monomeric Ru(II1) complexes with similar ligands, and the effect occurs a t much lower temperatures than those observed here.'" The simplest model for analysis of S = spin-coupled dimers is the so-called Bleaney-Bowers e x p r e ~ s i o n : ' ~ ~
= (2NPP2/3kT)[1
- Y3 exp(-U/kT)l-'
(2) Here J is the exchange coupling constant, g is the powder-averaged g value, and 6 is the Bohr magneton. This model neglects orbital angular momentum contributions, which are important for Ru(HI),tqs. Drillon et alazohave developed procedures for including the orbital contribution for t2 " configurations, but analytical expressions are not available. donetheless, the use of eq 2 should provide a good first approximation for J. Thus,the data of Figure 3 were analyzed by using eq 2 and included a Curie-Weiss term, x = C / ( T - e),to model the sharp upturn seen at low temperatures ascribed to a paramagnetic impurity. The fit, the solid line in Figure 3, is excellent, yielding the parameters J / k = -77 K, g = 1.76:' C = 0.047 emu cm-3 K-I, and 8 = -2.2 K. It is interesting to contrast the antiferromagnetic behavior found in complex 1 with the absence of significant magnetic interactions (14
